The revelation that the National Security Agency has been secretly amassing huge amounts of data about Americans’ phone and Internet use has sparked a lively debate about the proper role of secret information in a free and open society.

The crux of the debate is whether the value of secret information justifies the sacrifice of personal privacy. If secret information yields valuable intelligence that can be used to protect Americans, the reasoning goes, then it is worth sacrificing privacy for security.

But there is a major problem with evaluating information labeled “secret”: people tend to inflate the value of “secret” information simply because it is secret.

In a recent series of studies that we will present in a forthcoming issue of the journal Political Psychology, we have shown that people apply what we call a “secrecy heuristic” — a rule of thumb, in other words — when evaluating the quality of information related to national security. People rate otherwise identical pieces of information as more accurate, reliable and of higher quality when they are labeled secret rather than public. And people tend to think that national security decisions are wiser and better-reasoned when based on the same information labeled secret rather than public.